Glycofurol (tetraglycol) has been used in several countries as a solvent for drugs administered parenterally, although it has not been cleared for use in the U.S.A. When administered i.p. (1 ml/kg 2X/day for 4 days) to male Sprague-Dawley rats (150-200 g), glycofurol increased significantly in liver microsomes the concentrations of cytochrome P-450 (65%), cytochrome b5 (34%), NADPH-cytochrome c reductase (86%), and the activities of ethylmorphine demethylase (51%) and benzopyrene hydroxylase (74%). Glycofurol treatment decreased hexobarbital sleeping time (60%). Upon awakening, plasma and brain levels of hexobarbital were slightly (less than 22%) lower in treated than in control rats. A reduction (30%) in zoxazolamine paralysis time was correlated with an increase (38%) in liver microsomal metabolism and substantial decreases in plasma (55%) and brain (37%) zoxazolamine levels upon recovery of the righting reflex. We conclude that glycofurol treatment induces hepatic microsomal enzymes.